The invention relates generally to monitoring a channel in a telephone system, and in particular to a method and apparatus for monitoring DDS loop performance between the customer equipment and a office channel unit (OCU).
Dataphone Digital Service (DDS) has been widely available for sometime in the United States. The local loop of Dataphone Digital Service employs a communications protocol identified as alternate mark inversion (AMI) and provides digital signals between the customer premises and the local office, typically to an office channel unit (OCU). The communications use alternate mark inversion to provide a zero average current on the loop. This provides, therefore, an absence of net current flowing in either direction.
In accordance with the alternate mark inversion protocol, digital signals are converted from a "one" and "zero" format such as is found in typical digital electronic circuits to a format where a one bit is transmitted as a pulse, either positive or negative, while a zero is transmitted as no pulse. The pulses alternate on the line in order to maintain the zero average current flow across the line.
In circumstances where there are channel errors, which can be caused, for example, by electrical noise on the line, or equipment which is operating improperly, there results a line fault which can be serious not only to that particular line, but to other lines as well. In particular, when the DDS loop is operated as part of a multipoint circuit, that is, when several customers share a common communications channel using a multipoint unit located at the central office, one customer to central office DDS loop, producing incorrect signals, can adversely affect other customers coming into the same junction point. This situation is known as streaming branches, and the duplex path from the multipoint junction unit to the customer's control unit is called a branch.
Since errors on the system from one customer may adversely affect the operation of the multipoint Junction unit with regard to other customers, it is important to ensure that such errors are caught early. For this reason, performance monitoring equipment has been developed and used which provides certain levels of reliability in monitoring the performance. Such equipment typically attempts to recognize and distinguish between so-called intentional bipolar violations and unintentional bipolar violations. It is desirable to consider an alternate mechanism for detecting errors on the channel which measures and quantifies the current itself, and signals when a zero average current condition no longer exists.